U.S. Pat. No. 4,215,548--issued Aug. 5, 1980, for a "Free-Piston Regenerative Hot Gas Hydraulic Engine", discloses a free-piston Stirling engine construction, which, while it is not a resonant free-piston Stirling engine, is highly instructive as to the measures which have been undertaken with respect to free-piston Stirling engines in order to control their operation and control output power. The engine described in U.S. Pat. No. 4,215,548 requires the use of an external drive system for the displacer and this external drive system may employ pneumatic, electromagnetic or hydraulic sub-systems to provide the externally applied driving forces for the displacer. FIG. 4 of U.S. Pat. No. 4,215,548 illustrates an embodiment which employs an electromagnetically operated solenoid for applying the external forces for driving the displacer.
The present invention is to be contrasted to the free-piston Stirling engine disclosed in FIG. 4 of U.S. Pat. No. 4,215,548 in that it is a resonant free-piston Stirling engine wherein at least partial displacer power is derived from the thermodynamic cycle of the engine, and the output power derived from the engine is controlled by adjusting the displacer phase angle relative to the phase angle of the working member (power piston) of the engine by either applying power to or extracting power from the displacer externally.